1. Field of the Invention
This invention relates to pneumatic radial tires for heavy vehicles and more particularly to improving the heat resistance and durability of pneumatic radial tires used at high speeds under heavy loads.
2. Description of the Related Art
In recent years there has been an increasing tendency to use radial tires in high-performance vehicles due to improved road conditions. This tendency can also be attributed to the coincidental social demand for conserving energy and the cost savings associated therewith. The desirability of cost savings naturally has led to the demand for lightweight tires. For achieving this end, tires have been developed employing polyester cords as reinforcing materials instead of steel cords. Furthermore, radial tires employing reinforcing cords made of aromatic polyamide fibers such as Kevlar manufactured by DuPont have also been recently commercially developed. The tires employing the above-described reinforcing cords can be inflated to a high pressure and can safely maintain such a high inflation pressure.
However, the molecular structure of the aforesaid aromatic polyamide fibers is so highly oriented that their adhesion to other materials is poor and their resistance to flexing is inferior. When the aromatic polyamide fibers are used as a carcass of ply cords in a tire, then there is a tendency for an abrupt deterioration of the durability of the tire, in that the tire tends to separate or burst during service. On the other hand, when polyester cords are used as a carcass of ply cords, then the problems associated with aromatic polyamide fibers do not occur, but other problems arise such as the polyester tending to be decomposed by water and amine components contained in rubber. Therefore, polyester cords are not suitable for use as carcass of plies in a tire subjected to severe operating conditions. While there has been an increasing need for pneumatic radial tires for heavy vehicles which can be used under severe conditions such as at high speed, under a heavy load and during long distance travel, actual conditions under which tires are used are becoming severer.